1) Field of the Invention
The present invention relates to a laser diode module that can vary output light intensity, and in particular to such a laser diode module that can be reduced in size with a simple structure.
2) Description of the Related Art
A laser diode (LD) module includes a laser diode, a temperature regulating member like a Peltier element, a coupling lens, and an isolator that are incorporated in a housing (see, for example, Japanese Patent Application Laid-Open No. H7-301734). In recent years, an increase in capacity and an increase in speed have been promoted for optical communication. The LD module is used as light sources for respective channels of Wavelength Division Multiplexing (WDM) in the optical communication.
Such a LD module provided in each of the channels in the WDM communication is required to adjust output light intensity for each of the channels. Conventionally, a variable optical attenuator (VOA), which is externally attached to the LD module, has been used for adjustment of light intensity of light outputted from the LD module (see, for example, Japanese Patent Application Laid-Open No. 2001-13477 and Japanese Patent Application Laid-Open No. H11-52339). The VOA described in Japanese Patent Application Laid-Open No. 2001-13477 is a single VOA using liquid crystals. The VOA described in Japanese Patent Application Laid-Open No. H11-52339 is a VOA in which a polymer dispersed liquid crystal layer is inserted between fibers. Besides, a method of adjusting output light intensity using both control for a bias current supplied to a LD and control for laser chip temperature has also been proposed (see, for example, U.S. Pat. No. 6,563,846).
FIG. 11 is a diagram of a structure for adjusting output light intensity in a conventional technique. The structure shown in the figure is for one channel in the WDM communication. As shown in the figure, output light of an LD module 100 is connected to a VOA module via an optical fiber 101 and is made incident on an optical fiber 103 of a single mode (SM) or the like with output light intensity thereof adjusted by a VOA 102.
However, for example, if the VOA 102 of the structure described in Japanese Patent Application Laid-Open No. 2001-13477 is externally attached to the LD module 100, problems like an increase in loss due to insertion of the VOA 102, an increase in cost due to an increase of the number of components or the like, and time and labor required for adjustment are caused. There is also a problem of an increase in an installation area due to the external attachment.
For example, in the structure shown in FIG. 11, when emitted light from the LD module 100 due to linear polarization is made incident on the optical fiber 101 once, optical systems 110a, 110b, and 110c like a lens or a polarizer for aligning polarization states between the LD module 100 and the optical fiber 101 are required on an emission side of the LD module 100 and an incidence side of the VOA module 102. Light emitted from the LD module 100 is linear polarized light. When this light is made incident on the optical fiber 101 once, since a polarization characteristic of the optical fiber 101 is not always identical with that of the VOA 102, an optical system or the like for making the light made incident on the VOA 102 independent of polarization is required.
In the structure described in Japanese Patent Application Laid-Open No. H11-52339, it is necessary to increase a thickness of a liquid crystal layer in the polymer dispersed liquid crystal layer to obtain a sufficient amount of attenuation. Thus, there is a problem in that a high voltage is required and hysteresis occurs, making it impossible to obtain an accurate amount of attenuation.
A method of adjusting output light intensity of a laser module by controlling a bias current of a LD has also been proposed. However, a wavelength of output light is shifted simply by controlling the bias current. Thus, in particular, when the method is applied to an LD module based on a frequency grid of ITU-T of Dense Wavelength Division Multiplexing (DWDM), there is a problem in that the light with the shifted wavelength deviates from a frequency grid set for each short frequency interval.
In the method described in U.S. Pat. No. 6,563,846, there is a problem in that control for a bias current supplied to the LD and a laser chip temperature is complicated.